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We analyze two substorm onset lists, produced by different methods, and show that the (B-x . v(z)) product of the solar wind (SW) velocity and interplanetary magnetic field (IMF) components for two thirds of all substorm onsets has the same sign as IMF B-z. The explanation we suggest is the efficient displacement of the magnetospheric plasma sheet due to IMF B-x and SW flow v(z), which both force the plasma sheet moving in one direction if the sign of (B-x . v(z)) correlates with the sign B-z. The displacement of the current sheet, in its turn, increases the asymmetry of the magnetotail and can alter the threshold of substorm instabilities. We study the SW and IMF data for the 15-year period (which comprises two substorm lists periods and the whole solar cycle) and reveal the similar asymmetry in the SW, so that the sign of (B-x . v(z)) coincides with the sign of IMF B-z during about two thirds of all the time. This disproportion can be explained if we admit that about 66% of IMF B-z component is transported to the Earth's orbit by the Alfven waves with antisunward velocities.

Plain Language Summary We explore the interplanetary magnetic field configuration together with the direction of the solar wind flow velocity to demonstrate that the plasma sheet motion, which is caused by both magnetic field component B-x and solar wind flow component v(z), may affect the substorm onset. The obtained statistical result shows that the number of substorms, which broke in a situation when IMF Bz has the same sign as (B-x . v(z)) (favorable for the plasma sheet displacement), is 30-50% larger than the number of other substorms. The same relation was obtained for two different substorm onset databases, one obtained from Imager For Magnetopause-to-Aurora Global Exploration Far Ultraviolet observations and the other from ground-based AL index.